1. Field of the Invention
The present invention relates to optical recording media and optical-recording-medium playback apparatuses for playing back information recorded on the optical recording media. More specifically, the present invention relates to an optical recording medium having a plurality of recording layers and an optical-recording-medium playback apparatus for playing back information recorded on the optical recording medium.
2. Description of the Related Art
Optical-recording-medium playback apparatuses which read out (playback) information recorded on optical recording media, such as compact discs (CDs) and digital versatile discs (DVDs), by optical means are known in the art. In such an optical-recording-medium playback apparatus, a recording layer of an optical recording medium is irradiated with light, and information recorded on the optical recording medium is read out by detecting the intensity, etc., of light reflected by the recording layer. In addition, the optical-recording-medium playback apparatus may also be used for recording information on the optical recording medium as necessary.
In some optical recording media, a plurality of recording layers are provided so as to achieve a large capacity. For example, DVDs have two recording layers while CDs have only one recording layer. In accordance with the increase in the capacities of optical recording media, the number of recording layers is expected to increase further.
However, as the number of recording layers increases, it becomes more difficult to completely separate signals representing information recorded on different recording layers. When, for example, a recording layer from which information is to be read out is irradiated with light in an information readout operation of an optical recording medium, this light also passes through recording layers other than the one from which information is to be read out. Therefore, the light reflected by the optical recording medium includes light reflected by the recording layers other than the one from which information is to be read out. This additional light (stray light) increases as the number of recording layers increases, and functions as noise in the information readout operation. Accordingly, a signal-to-noise (S/N) ratio of a playback signal is decreases.
On the other hand, optical-recording-medium playback apparatuses using near-field light have recently been attracting attention. In particular, in optical-recording-medium playback apparatuses using solid immersion lenses (SILs), the focal spot size of a light beam can be reduced since the SILs have a high numerical aperture (NA), and a high recording density can be achieved. In addition, the SILs are highly compatible with conventional optical disc systems, and optical-recording-medium playback apparatuses using the SILs are therefore expected to become increasingly popular.
However, there is a limit to increasing the memory capacity of the optical-recording-medium playback apparatuses using the SILs. This is because since the recording density is determined depending on the wavelength of light, it is difficult to increase the recording density beyond a certain limit.
In addition, tracking must be performed in order to playback the information recorded on the optical recording medium, and it is necessary to form grooves or the like for tracking on the optical recording medium.
The grooves for tracking are preferably formed on each of the recording layers, and the grooves are required to have a certain depth, for example, about λ/6 in a push-pull method and about λ/4 in a differential phase detection (DPD) method (λ is the wavelength of a light source).
Accordingly, the interval between the recording layers must be increased to some extent, and there is a possibility that this will be a barrier to the development of optical recording media having multiple recording layers.